It was about 6 years ago when the first cracks in the armor — in this case, Bt corn — were discovered in Iowa.

And today, the threat of Bt resistance in corn hasn’t really lessened, as it’s been detected in many other areas of Iowa and several other Midwestern states, particularly in the western corn belt.

No-tillers wanting to preserve the effectiveness of Bt corn, and their yields, will need to start varying their pest-management approaches, rather than relying on the same Bt trait every year, says Purdue University entomologist Christian Krupke.

“If you were conditioned to think about Bt in terms of the corn borer Bts — which are as close to a silver bullet as we’ve ever seen in field crops — we’re not going to have that story here,” he says. “But it’s not all bad news.”

Double Trouble.

Krupke says Bt resistance has emerged for one key reason: because many growers have planted the same Bt traits continuously for several years, ramping up selection pressure.

Krupke says refuge-in-a-bag programs were supposed to bring Bt and non-Bt beetles together in closer space to mate and slow the spread of resistance beetles.

“But it doesn’t happen that way in biology,” he says.

Adult beetles that emerge from Bt corn often show a lag in development, which delays their natural emergence pattern compared with adults that emerge after feeding on refuge corn as larvae.

Refuge male beetles may not wait for Bt females to emerge, and will mate…